1. Field of the Invention
The present invention relates to devices useful for pumping fluids and semisolids. More particularly, the present invention relates to devices such as double diaphragm pumps which are driven by a fluid.
2. Description of the Invention's Background
Various devices have been developed which are useful for pumping fluids or semisolids and which are driven by some type of a fluid such as air. Many of such devices which use air, compress the air during a portion of the pumping cycle and then exhaust the compressed air to atmospheric pressure. If there is water vapor in the air, i.e., humidity, and it is not removed from the compressed air before it enters the pump, the cooling effect of polytropic, adiabatic expansion of the compressed air as it is exhausted can cause the water to freeze. As an example, if the relative humidity of the air is 40 percent and a volume of that air is compressed to one half of its original volume, the relative humidity of the air becomes 80 percent because the volume of the water does not significantly change. The temperature drop caused by adiabatic expansion of the compressed air from a pressure of 4.5 bar (approximately 65 psi) to atmospheric pressure, at a room temperature of 68 degrees Fahrenheit, is about 120 degrees Fahrenheit. Thus when the air undergoes rapid adiabatic expansion, i.e., expansion without the addition of heat, the temperature of the air drops quickly and the moisture in the air freezes. When the moisture freezes it tends to build up in and block an exhaust passage of an air driven pump, and it eventually can completely shut off the exhaust passage, preventing operation of the pump. The temperature reduction can be so great that not only will the water vapor in the exhaust air freeze, but also the housing of the pump can become so cold that water vapor in the atmosphere will condense and freeze on the exterior of the pump.
Various air driven pumps have accordingly been designed which include some means for reducing the freezing of water vapor entrained in the air which drives the pump, or for reducing blockage of an exhaust passage of the pump due to freezing of the water vapor. For example, U.S. Pat. No. 4,566,867 to Bazan, et. al. discloses a pneumatically operated reciprocating three way valve of a double diaphragm pump, which includes a mechanism to reduce ice in the valve. As shown in FIG. 3, a needle valve allows the controlled bleeding of high pressure air from an internal high pressure chamber to an internal low pressure chamber. The high pressure air is disclosed as furnishing internal energy, i.e. velocity, to the exhaust air to mechanically displace ice as it forms. U.S. Pat. No. 4,921,408 to Kvinge et al. discloses a silencing system for an air operated pump which is asserted to also eliminate icing of the pump at higher cycle rates and humidities. Exhaust air exiting from an air cylinder is mixed with relatively warm ambient air in an air flow inducer such that they form a mixed flow in an exit stream. The exit stream has a substantially lower velocity and higher temperature than the air leaving the exhaust in the exhaust nozzle block, and thus icing is assertedly reduced. U.S. Pat. No. 2,944,528 to Phinney discloses an air distributing valve which includes a reed which is oscillable in an exhaust port of the valve. A cavity adjacent the exhaust port has a coating, such as silicon resin or Teflon.RTM., which tends to prevent ice from adhering thereto. The reed and the coating are disclosed as cooperating to prevent ice formation.
U.S. Pat. No. 4,406,596 to Budde discloses a double diaphragm pump which includes a mechanism for equalizing the pressure between two air chambers. The pressure equalization reduces the pressure blow off that occurs in the exhaust of air from the pump, and thus is stated to reduce the danger that ice will form at the air exhaust. U.S. Pat. No. 3,176,719 to Nord, et al. discloses a four-way valve which attempts to minimize ice formation in exhaust ports by eliminating impediments in the exhaust passages on which ice may form. Also, resilient washers are provided in the exhaust ports and these washers are contacted by closures during the valve cycle to break loose ice which may form on the washers. U.S. Pat. No. 3,635,125 to Rosen, et al. discloses a double-acting hydraulic pump and air motor which includes a muffler. Ice accumulation in exhaust spaces of the muffler is avoided because an outer plate of the muffler is flexible, and when ice accumulates in the spaces the resultant increase in exhaust pressure causes flexure of the plate which causes blowout or purging of the ice from the spaces.
The devices disclosed in the patents cited above each utilize either some type of air mixing or some type of moving element to attempt to reduce ice formation therein. The ice reduction mechanism of each of the disclosed devices thus has the disadvantage that it adds to the overall complexity of the device's design by adding either additional air flow paths, additional moving parts, or both to the device, which can contribute to greater manufacturing costs and repair downtime for the device.